The present invention relates to a method for igniting and generating an expanding, diffuse microwave plasma. The invention furthermore relates to a device for performing such a method. The method is suitable for generating microwave plasmas for the purpose of plasma-treating surfaces and substances, in particular three-dimensional objects and also particles under atmospheric pressure.
Microwave plasmas are well suited for performing various plasma treatments, such as e.g. activation, cleaning, coating, sterilization, modification, and functionalization of surfaces. The use of a diffuse, largely homogeneous, expanded plasmas is desired for this.
In known methods (DE 4235914 A1, EP0209469, DE19726663), such plasmas are preferably ignited and generated in the low pressure range or near atmospheric range. Although it is possible raise it and use it in the normal pressure range, this renders the plasma treatment process very sensitive and unstable. Minor changes (e.g. from gas flow, mixing in process gases, and mixing in aerosols and particles) causes the plasma to extinguish, and it must be re-ignited, which is very complex.
Other known methods for plasma treatment under atmospheric pressure, such as barrier discharge, are not suitable for treating three-dimensional objects and highly structured surfaces.
Various embodiments of plasma jets (DE19605518, EP0968524, U.S. Pat. No. 5,798,146) require a high gas flow, generally a special working gas, for driving out the plasma, and are problematic in terms of ignition behavior. Moreover, they generate only a small volume of plasma having a small diameter. Thus they are not suitable for large-scale applications and are expensive in terms of production and operation.
Thus a method for igniting and generating a spatially expanded plasma in normal pressure or high pressure and having high ignition certainty, stable operation, and the lowest possible gas throughput would be advantageous.
Wider and more cost-effective use of the plasma treatment is not possible in many fields unless complex vacuum technology is not needed, working gases and process gases are used less, and handling is simple and safe.